Yeast extracts are widely used as flavoring agents for soups, meat-based products, sauces, for example. Among the key flavor components of these extracts are especially guanosine 5'-monophosphate (GMP) and inosine 5-monophosphate (IMP), the latter being a major nonvolatile compound used to enhance the flavor of meat-based products. IMP is derived from the oxidative deamination of adenosine 5'-monophosphate (AMP). This reaction is catalysed by an enzyme called adenosine monophosphate deaminase which is found in many eukaryotic or prokaryotic microorganisms, and especially in certain moulds of the genus Aspergillus.
JP 53018773 describes the conversion of the AMP of a yeast lysate to IMP by an adenosine monophosphate deaminase isolated from an Aspergillus niger from a koji culture.
JP 55120788 also describes the presence of an adenosine monophosphate deaminase in certain halophilic microorganisms, especially Torulopsis or Candida. Unfortunately, this deaminase exhibits thermal instability from 40.degree. C. and a relatively low enzymatic activity, which makes its industrial use particularly difficult.
In addition, it is known that the method for the preparation of a traditional soya sauce has two fermentation stages involving an Aspergillus and a halophilic mircoorganism respectively. EP 417481 thus describes a process for the preparation of a fermented soya sauce, in which a koji is prepared by fermentation, with a koji culture, of a mixture of cooked soya and roasted wheat, the koji is hydrolysed in an aqueous suspension for 3-8 h at 45.degree.-60.degree. C. with the enzymes produced during the fermentation with the koji culture, a moromi is prepared by adding sodium chloride to the hydrolysed koji suspension, the moromi is fermented by a halophilic microorganism, it is pressed, a liquid is recovered from the pressed moromi or cake, this liquid is pasteurized and it is clarified in order to remove the sediment.